In the manufacture of printed circuit assemblies electrical pin contacts are inserted into multilayer boards called backpanels. It is desirable to retain the structural integrity of the board, while guaranteeing electrical communication between the pin and board.
The backpanel itself is in a sandwich construction of a layered matrix of conductive pathways between sandwiched layers of dielectric material. Electrical communication with the conductive pathways occurs by means of stamped and plated pins, which need a hole through the backpanel in which to make such contact.
Such pins also constitute support points for daughter boards and other similar electrical components. The pins hold these daughter boards, and also constitute electrical communication with them. In the modern digital environment, these electrical contacts must be uniform, and cannot have unacceptably variable resistances. The traditional method of manufacture works against and in opposition to these dual requirements of support and electrical communication.
A complete description of how a board has a pin mounted to it can be simply stated. A board is drilled out at precisely positioned points, so that some of the conductive pathways are pierced, while others are avoided. These drilled holes are then subjected to a plating process which leaves resident a plated cylinder.
The electrical connection between the conductive layer at the drilled interface and the material plated into the drilled hole must be carefully understood. This juncture is a very sensitive and delicate interface of electrical connection which must not be disturbed or interfered with. Movement of the plated cylindrical aperture, either by shearing or twisting, or moving the conductive layer out of its planar disposition can cause interference with the desired electrical characteristics of the pin. In short, this delicate electrical interface between plated cylindrical hole and conductive layer must be substantially undisturbed.
Pins are inserted in the holes in a forcefitted process. Because of the nature of the plating process, the resultant holes are not precisely sized, but have varying diameters.
Therefore a pin connector is needed that is both insertable in holes of slightly varying diameters and has the structural integrity required to support daughter boards as well as withstand the insertion force. In addition, it is desirable that the pin provide uniform electrical connections.
So-called "eye of the needle" compliant pins are known. These pins at all stages of deformation maintain a spatial separation between their respective compliant legs. Unfortunately, these compliant legs are subject to a kind of columnar collapse. This columnar collapse pulls the pins out of electrical contact with the plated hole into which they are inserted. This is highly undesirable.
When a pin separates from the sides of a plated cylindrical hole, the connection of the pin to the hole is no longer "gas tight". When the connection is no longer gas tight actual corrosion of the pin in the field has been known to occur. The back panel fails in the field and frequently has to be replaced. Since the back panel typically is the very foundation of the electrical equipment into which it is placed, such failures can be catastrophic.
Other compliant pins are known. Such pins typically have a solid pin member at each end and two legs joined at said ends, with a sliding interface between said legs.
We have found relative to the prior art pins that two major faults cause unsatisfactory connections. First, if the legs exhibit a cross-section which includes a sharp edge that contacts the aperture, the pins ride into and deform the cylindrical holes into which they are placed. This deformation alters and changes the desired conformity of the electrical connection. It can be seen by photographic analysis that the sides of the cylindrical holes are also destroyed.
In addition, we find that such pins undergo twisting, either of the entire pin or portions thereof, during the insertion process. This defect causes the pin to make non-uniform electrical contact. It also tends to destroy the cylinder itself, at the points where it makes the required electrical connection.
Nonuniform contact between the plated hole and the pin or deformation of the hole (especially deformation which destroys the planar disposition of the electrical conductive layer) charges the impedance of the electrical connection. This change in impedance may or may not relate to the integrity of the electrical connection; it nonetheless destroys the utility of the circuit.
Deformation of the cylinder, with the resultant whole or partial destruction of the required electrical connection, may necessitate rejection of the entire backpanel, since it is often difficult if not impossible to determine where the fault exists in the circuit.
Some prior art pins have their respective legs separated one from another by "lancing". In such a process, the metal of one leg is abruptly sheared from the metal of an adjoining leg. In actual photographs we have taken it can be seen and we have discovered that the interfaces produced by such shearing are rough or at least microscopically serrated. These rough edges are believed to produce adverse compliant spring forces when the pins are inserted. As will be understood, we go to the expedient of working and polishing by stamping to avoid this irregular interface produced by shear.
We also note that certain prior art pins have their respective legs totally offset and bent out of the plane of the material from which they were originally formed. Such pins, in order to compliantly yield, must restraighten the bent legs. Resistance increases as the respective legs approach their original coplanar disposition. Such pins have an increasing tendency to deform and destroy the cylindrical apertures into which they are placed.
The reader will understand that the discovery of the problems of the prior art as well as their solution can constitute invention. Consequently, we herein state the difficulties with the prior art as we have come to know them after extensive experimentation. These difficulties may at best be divided into defects which are inherent to the pin itself, as well as defects that the pin imparts to the board which it transverses.
With respect to the pin, where it requires excessive force being inserted into the hole, difficulties occur. Buckling as well as hole destruction are some of the effects.
Further, the pin, when in the hole, can have a low retention characteristic. With such a characteristic not only is the pin easily removed, but the electrical connection can fail.
Likewise, the pin can be subject to cracking, this cracking especially occurring where compliant legs depart from the main body of the pin. Such departure interferes with the structural integrity of the pin and renders nonuniform the desired electrical connection.
Likewise, the pin can be bent either during the insertion process itself or as the pin protrudes from the hole. Where a matrix of such pins are required for the connection of components, a pin out of align prevents the structural attachment.
Further, it is known that pins twist during insertion. This twisting force can be a source of loss of the desired electrical connection. Further, the pin itself can be canted or cocked as it protrudes from the board. This canting or cocking of the pin prevents the desired connection of electrical components.
Likewise, where the legs of the pin are separated from the sidewalls of the plated hole, a non-gas tight connection occurs. This non-gas tight connection can be the subject of corrosion, which corrosion eventually destroys the electrical connection of the pin and the utility of the backpanel of which it is a part.
Improper pin design also causes difficulty with the plated through hole. Specifically, the pin in passing can gouge and create plating voids. These voids interfere with the uniform impedance required for modern digital electrical connection.
Most crucially, if the plated cylinder is destroyed at or near an electrical connection to one of the conductive layers of the board, critical damage can be done.
Further, tight pin fitting causes slivers to be dislodged and fall, not only across the plated hole, but elsewhere throughout the backpanel assembly. There results slivers which can lodge and cause undesired short circuits, which circuits are extremely difficult to restore to their intended dielectric condition.
Improperly fitted pins cause change of electrical impedance values in the board. While the change of these electrical impedance values is difficult to quantify, it can come from changed resistance, electromagnetic forces extending between adjacent conductive layers, disturbed portions of the plated hole and many other factors which in the microscopic environment of the board are difficult to identify.
It should be realized that when groups of pins with improper fit are inserted, they can together cause warping and bowing of the backpanel board. Since the backpanel board frequently provides the very foundation of the electrical component of which it is a part, such warping and bowing is unacceptable.
Likewise, the pins must be capable of some working as they form the desired interconnection with the board. Lack of this working can cause delamination of the board in the field, again resulting in board failure.
We have found that an inordinate amount of attention has been devoted to the function of the pins themselves; we have found that their interaction with the board is just as important.
We have found after two years of experimentation that pins forced in conductive holes and backpanels are very empirical and arbitrary in their performance. We therefore disclose in the following specification a specific design developed by us which we have found has a low incidence of failure of electrical connections and provides necessary flexibility for insertion into holes of a wide range of diameters, making for all this range the required electrical connection.